The present invention relates to an apparatus for detecting misfiring in an internal combustion engine, and more particularly to a misfire detecting apparatus for an engine of a motor vehicle.
Misfiring is a phenomenon which occurs in an internal combustion engine when a cylinder of the engine fails to ignite. Misfiring can be due to a number of causes, such as a failure of the ignition system to generate an adequate spark in a cylinder or a failure of a fuel supply system to supply a proper air/fuel mixture to the cylinder. When misfiring is due to a failure of the ignition system, uncombusted fuel is discharged from a misfiring cylinder. The discharge of uncombusted fuel from an engine is of course undesirable because of possible damage to a catalytic converter of the engine and for other reasons. Therefore, it is desirable to be able to detect occurrence of misfiring in an engine and then cut off the supply of fuel to the cylinder in which misfiring has occurred.
Under the circumstances, various misfiring detecting methods and apparatuses have heretofore been proposed and developed for practical applications. As one of such misfiring detecting methods, there may be mentioned a method disclosed in Japanese Published Unexamined Patent Application No. 58-19532 (JP-A-58-19532), according to which the angular velocity of a crankshaft of an engine is monitored, and decision of the occurrence of misfiring is made when a difference in the angular velocity measured at angular positions located before and after top dead center in the compression stroke exceeds a preset value. According to another misfiring detecting method such as described in JP-A-63-263241, decision as to the occurrence of misfiring is made on the basis of an air/fuel ratio determined by sensing, for example, an oxygen content in an exhaust gas of the engine.
The misfire detecting methods and apparatuses known heretofore, however, suffer from a common problem that reliability in misfiring detection is poor. In the case of the first mentioned misfiring detecting method, for example, a change in the angular velocity of the crankshaft brought about by misfiring is often too insignificant to detect the occurrence of misfiring with reasonable reliability particularly when the engine is operated at a high speed in a unloaded state or in a light load state, because of a low friction loss and high inertial energy of the engine in these operation states. Besides, the angular velocity of the crankshaft of an engine installed on a motor vehicle is likely to change due to other causes than misfiring such as shocks and vibrations to which the motor vehicle is subjected when running on a rough or bad road. On the other hand, in the case of the second mentioned misfiring detecting method, misfiring can be detected only with a delay due to a time lag intervening between the occurrence of misfiring and arrival of the uncombusted gas mixture at the location where the air/fuel ratio sensor installed. Furthermore, the air/fuel ratio is susceptible to changes due to other causes than misfiring. By way of example, the air/fuel ratio considerably varies upon acceleration and deceleration of the engine. Additionally, in a multi-cylinder engine, the air/fuel ratio may differ from one to another cylinder. For these reasons, changes in the output of an air/fuel ratio sensor brought about by the other causes may erroneously be detected as a symptom of misfiring. In other words, the misfiring detecting methods and apparatuses known heretofore are disadvantageous in that misfiring detection can not be performed with satisfactory reliability.